Systems for cylinder deactivation have been proposed previously, in order to achieve improved fuel economy and reduced emission when the engine is operating at part load. Such systems rely on the fact that spark ignition engines operate less efficiently at low load because of the pumping losses caused by throttling. Especially in a large engine, it is more efficient to run one bank of cylinder under higher load than two banks under lesser load, while producing the same output power. Cutting off the fuel supply to one bank of cylinders achieves the desired reduction in fuel consumption but when the disabled cylinders are still allowed to pump air, this upsets the stoichiometry of the exhaust gases and interferes with the operation of the catalytic converter. The presence of excess air in the exhaust gases means that the catalytic converter cannot neutralise NO.sub.x present in the exhaust gases, as this requires a stoichiometric or reducing atmosphere. For this reason, known systems take special steps during cylinder deactivation to avoid air reaching the catalytic converter through the disabled cylinders. The steps that have been proposed for this purpose include maintaining the intake and exhaust valves of the disabled cylinders permanently shut, or running the disabled cylinders with 100% EGR. Both these proposals have disadvantages in that valve disablement is costly to implement and switching to 100% EGR gives rise to problems in controlling the combustion during the periods of changeover between normal operation and deactivation. Also, undesirable leakage of EGR gases into the intake system of firing cylinders is difficult to avoid.
JP-A-55 029002 discloses an engine have two groups of cylinders 1-3 and 4-6. Under high low, both groups of cylinders are operational but under light load, cylinders 1-3 are disabled. The exhaust gases from both groups of cylinder pass through a main catalytic converter which is preceded by a first oxygen sensor. The exhaust gases from cylinders 4-6 additionally pass through another catalytic converter arranged upstream of the common catalytic converter and itself preceded by a second oxygen sensor. The first oxygen sensor sets the fuel quantity during high load operation and the second sets the fuel when only one group of cylinder is firing. During part load, the main catalytic converter and the first oxygen sensor tend to cool down and the engine is forced to run on all cylinders for a short time to heat the main catalytic converter whenever its temperature is sensed by a detector to be dropping below a minimum threshold.
JP-A-55 49549 discloses an engine with two groups of cylinders that can be selectively deactivated during part load operation. The engine exhaust system has three catalytic converters one main converter common to both groups of cylinders and two further converters arranged upstream of the main catalytic converter, each associated with a respective one of the two groups of the cylinders. Each time that the engine is switched from running on both groups of cylinders to only one group, the group of cylinders selected for deactivation is alternated. As a result, neither group of the cylinders is allowed to run cold and neither group is subjected to wear at a different rate from the other group.